Field of the Invention
This invention relates to methods and compositions for polymer flooding operations. More specifically, the invention relates to methods and compositions using smart water and polymers for polymer flooding operations in carbonate reservoirs.
Description of the Related Art
Polymer flooding processes are one of the matured enhanced oil recovery (EOR) technologies and are currently gaining some importance for viscous oil recovery in carbonate reservoirs. Increased aqueous phase viscosities due to polymer addition decrease the mobility of injection water to provide some mobility control and mitigate viscous fingering with such crude oils in the displacement process. Polyacrylamide based polymers or co-polymers are the most widely employed polymers for EOR today in the industry. These polymers are anionic in nature and their viscosifying characteristics are hindered by salinity and the monovalent/divalent cations present in the makeup water. The negative carboxyl groups of partially hydrolyzed EOR polymers interact strongly with positively charged ions such as monovalent and divalent cations present in the makeup water. These salt ions bind tightly to the negatively charged carboxyl groups in the polymer chain to render a “coiled state” and prevent the elongation/swelling of polymer molecules in water for increased viscosibility. The divalent cations are much more detrimental when compared to monovalent cations due to their strong bridging effect with polymer chain and they can precipitate polymer from solution even at relatively lower ionic concentrations. As a result, EOR processes that employ high salinity water/seawater typically need much higher dosages of polymer as compared to low salinity water to achieve decent viscosities required for proper mobility control in viscous oil recovery processes. Therefore, the use of high salinity water is typically limited to situations in which the availability of high salinity water, such as seawater, is readily available. Such requirements of high polymer dosage can become cost prohibitive to apply polymer flooding technology in certain formations containing viscous crude oils.
In addition, the formation temperatures and in-situ water composition can impact the type of polymer chosen for EOR.
In conventional polymer flooding operations, the polymer concentration is typically 1000 ppm or greater in the injection fluid. The injection fluid is typically injected as a finite slug of at least 0.3 pore volumes for processes involving carbonate reservoirs and viscous oil recovery.